Analog to Digital Converter

Douglas Taylor dj.taylor4 at
Wed Aug 19 07:59:56 CDT 2015

On 8/18/2015 7:01 PM, dwight wrote:
> Reading the note made me think of something else.
> If it is an SAR, and you read to soon, the LSBs of the incomplete
> conversion will always be the same. All ones or all zeros.
> If it is a Sigma-Delta, and the input has noise
> on it that is above the same rate, the LSB's will take on a
> random, all ones or all zeros on different reading.
> This is an indication that the input has not been properly
> filtered or you have exceeded the slew rate, even if you are
> within the frequency band width.
> Most audio A/Ds assume a de-emphesis so that the higher
> frequencies are reduced in slew rate.
> Any noise above 1/2 the sample rate will be reflected into
> the values. Anything above the sample rate will be as I
> described, a mix of all 1's or all 0's on each sample.
> Dwight
That is exactly what I am seeing.  The signal the ADC is trying to 
capture looks like a sinc function, lots of small ripples with a narrow 
high amplitude burst in the center.  What gets lost are the small 
amplitude values.


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